1. Field of the Invention
The present invention relates to an amplitude control circuit, polar modulation transmission circuit and polar modulation method used in mobile telephones and communication devices using wireless LAN and such. In particular, the present invention relates to an amplitude control circuit, polar modulation transmission circuit and polar modulation method that operate with high efficiency and low distortion.
2. Description of the Related Art
Mobile telephones and communication devices using wireless LAN and such are required to ensure linear characteristics of transmission signals over a wide range of output levels and still operate with low power consumption. Further, a transmission circuit that operates with high efficiency and low distortion is used in such communication devices. A conventional transmission circuit will be explained below.
As a conventional transmission circuit, for example, there is a transmission circuit to generate transmission signals utilizing a modulation scheme such as quadrature modulation (hereinafter “quadrature modulation circuit”). Here, the quadrature modulation circuit is widely known and explanation will be omitted. Further, as a conventional transmission circuit to output transmission signals with higher efficiently and higher linearity than a quadrature modulation circuit, for example, there is the transmission circuit disclosed in Patent Document 1. FIG. 1 is a block diagram showing a sample configuration of conventional transmission circuit 10 disclosed in Patent Document 1. Referring to FIG. 1, conventional transmission circuit 10 is provided with amplitude phase extracting section 11, amplitude control section 12, phase modulating section 13, amplification section 14 and output terminal 15.
Amplitude phase extracting section 11 extracts amplitude data and phase data from input data. The amplitude data is inputted in amplitude control section 12. Amplitude control section 12 provides the voltage based on the amplitude data to amplification section 14. Further, the phase data is inputted in phase modulating section 13. Phase modulating section 13 performs phase modulation based on the input phase data and outputs the result as a phase modulation signal. The phase modulation signal is inputted in amplification section 14. Amplification section 14 amplifies the phase modulation signal according to the voltage provided from amplitude control section 12. The signal amplified in amplification section 14 is outputted as a transmission signal from output terminal 15. The output level of the transmission signal can be controlled by changing the output voltage of amplitude control section 12 and providing the resulting output voltage to amplification section 14. Thus, a scheme for dividing input data into amplitude data and phase data and performing modulation using these data, is referred to as “polar modulation scheme” or “polar coordinate modulation scheme.” Further, transmission circuit 10 that implements this scheme is referred to as “polar modulation circuit (or polar coordinate circuit)”.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-266351 (FIG. 9)
However, conventional transmission circuit 10 has a problem that the output noise of amplitude control section 12 is difficult to suppress. The reason will be described below. FIG. 2 shows the configuration of amplitude control section 12 in detail. In FIG. 2, DA converter 12-1 converts amplitude data, which is a digital signal, into an analog signal. Level control section 12-2 changes the output level of DA converter 12-1 according to transmission power information P (which is oftentimes represented by digital signals) showing the magnitude of average output power of is transmission circuit 10. Buffer 12-3 amplifies the output of level control section 12-2 and outputs the result to amplification section 14. By employing such a configuration, transmission circuit 10 can change the output voltage of amplitude control section 12 and control the output level of the transmission signal outputted from amplification section 14.
Here, amplification section 14 uses a high output amplifier, which is generally called a “power amplifier,” and, consequently, buffer 12-3 of amplitude control section 12 that drives amplification section 14 needs to be able to provide large current. Further, DA converter 12-1 of the amplitude control section performs DA conversion of amplitude data, which has a much wider band than IQ data used in quadrature modulation and such, and therefore requires a high-speed clock.
Therefore, when amplitude control section 12 is formed with IC chips, oftentimes, DA converter 12-1 and level control section 12-2 controlled by digital signals adopt low-voltage process that enables high-speed operations, and buffer 12-3 adopts high-voltage process that allows the use of large current.
However, generally, unlike the process that allows the use of large current, the process that enables high-speed operations has a tendency of reducing a maximum voltage. Consequently, it is difficult to increase the output amplitude, that is, it is difficult to ensure a wide dynamic range. Therefore, the gain in buffer 12-3 needs to be increased, and, as a result, there is a problem that the noise outputted from level control section 12-2 is amplified in buffer 12-3 and the output noise of amplitude control section 12 increases. The output noise of amplitude control section 12 is outputted to amplification section 14. Consequently, if noise is outputted to, in particular, the receiving band, a decrease in reception sensitivity occurs which raises a serious problem.